1. Field of the Invention
The present invention relates generally to a business form printing press which performs a sequence of operations of printing and processing, such as punching, perforating and so forth. More specifically, the invention relates to a tension control system in a business form printing press, which maintains a tension on a printing web at an appropriate level for avoiding registration error upon printing or processing.
2. Description of the Related Art
It is important for a business form printing press to maintain tension between a plurality of roller nips in a web feeding portion, a printing portion, a processing portion and a folding portion at appropriate values (the same value, or gradually increased values) during operation, and thus to avoid fluctuation of expansion of a web for achieving high precision in registration in printing and processing.
However, in the conventional business form printing press, the following problems have been encountered.
(1) It has not been possible to appropriately control relationships of tension by employing means for numerical setting thereof, or the like, in relation to variations of tensions on the web between the roller nips due to thickness of the web rolled on a cylinder and a drawing ratio determined by a finishing diameter of the cylinder. The prior art mentioned hereinabove is disclosed in Japanese Patent Examined Publication No. 4(1992) - 51456. PA0 (2) In the business form printing press, printing cylinders have to be changed every time upon variation of longitudinal size. Therefore, it is difficult to appropriately maintain a drawing ratio of an impression cylinder which is determined by a finishing diameter of the impression cylinder between the web feeding portion to the processing portion and thus to cause fluctuation of the drawing ratios between the nips. This results in variation of tension on the web per respective roller nip at every occasion of changing of the printing cylinder. PA0 (3) In case of printing using a dampening water, the web is easily expanded. Therefore, unless the tension acting on the web is lowered in comparison with that of the printing unit which does not use the dampening water, the longitudinal length of a printed area becomes shorter as dried. For this reason, the tension to act on the web has to be individually adjusted for each printing unit. However, the conventional business form printing press does not permit such adjustment. PA0 (4) In the processing portion and the folding portion, feeding of web is performed by friction by locally exerted pressure of a tension roller and a rubber roller to easily cause slip by penetration of air between the web and the roller during rotation. This is easily caused in a high speed range. Therefore, tension in the high speed range becomes unstable to cause registration error relative to the printed area in processing or registration error between respective processing units, such as a punching unit, a transverse perforating unit and so forth. PA0 (5) In the processing portion, it is desired to lower the tension acting on the web after a transverse perforating process, especially in the case where perforations are large to be easily cut or perforations are formed in a thin web. However, the conventional business form printing press does not permit adjustment of tension, individually.
In such circumstances, it has been desired to permit adjustment of the tension to act on the web between rollers in the printing portion, the processing portion and the folding portion, individually.